In a sliding spool type valve, the valve body is provided with a plurality of valve ports including an inlet valve port, controlled outlet ports and return ports at spaced locations along the valve bore adapted for connection to different flow conduits, with annular valve seats between the adjacent ports cooperable with spaced lands on the sliding valve spool which controls flow between the several valve ports. In order to provide a satisfactory sliding spool type valve, it is necessary to accurately control the size, shape and spacing of the several valve ports along the valve bore with relation to the size and spacing of the lands on the valve spool. It is the common practice to form the valve bodies for such sliding spool type valves by casting the valve body with port recesses at spaced locations along the valve bore and thereafter machining the port openings utilizing an expansible tool that is inserted into the valve bore and which is expanded laterally of the valve bore at the appropriate locations therealong to machine the several valve ports. However, this machining operation is relatively slow and time consuming and even small imperfections in the machining of the valve ports, such as caused by scratches or nicks in the edge where the valve ports meet the valve seats, can cause leakage and erratic operation of the valve.
It has heretofore been proposed, as disclosed in the U.S. Pat. No. 3,598,152, to T. D. H. Andrews dated Aug. 10, 1971, to form a sliding spool type valve utilizing a plurality of relatively thick plates which are machined on the side surfaces prior to assembly to provide ports and flow passages which open laterally of the valve body. Such a valve construction, however, requires a number of differently machined and shaped plates to provide the several ports and passages. Moreover, that valve construction utilized relatively thick plates with passages that opened laterally of the valve body and additionally required substantial machining and assembly operations after the plates of the valve body were joined together, in order to provide a mounting pad and connection for the several flow passages.